A solid state lighting apparatus is typically designed to be driven with a DC power signal. However, electrical power is commonly delivered using an AC voltage signal, typically 110 to 220 volts at 50 to 60 Hz. Thus, a power converter is typically used to convert an AC power signal to a DC signal that can be used to drive a solid state lighting apparatus.
In general terms, power converter circuits may be used to convey power from a source, such as a battery, electrical power grid, etc. to a load, such as any device, apparatus, or component that runs on electricity, preferably with as little loss as possible. Generally, a power converter circuit provides an output voltage that has a different level than the input voltage.
One type of power converter circuit is known as a switching or switched mode power supply. A switched mode power supply controls the flow of power from a power source to a load by controlling the “on” and “off” duty cycle of one or more transistor switches in order to regulate the DC output voltage across the output terminals of the power supply. The “on” and “off” duty cycle of the one or more transistor switches may be controlled in response to a pulse-width-modulated (PWM) gate drive signal provided by a switching regulator circuit, such that the “on” and “off” duty cycle of the one or more transistor switches is determined by relative pulse-widths of the PWM signal.
Switched mode power supplies have been implemented as an efficient mechanism for providing a regulated output, and are generally more power efficient than linear voltage regulators, which dissipate unwanted power as heat.
Some switched mode power supplies may use a transformer or an inductor as an energy transfer element and a capacitor as an energy storage element. A power transistor may be coupled to one side of the primary winding of a transformer, and may be turned on and off in response to the gate drive signal provided by the switching regulator circuit to alternately store energy in the magnetic field of the transformer and transfer the stored energy to the secondary winding. The secondary winding of the transformer may develop a rectified output voltage across a shunt output capacitor, which is typically an electrolytic capacitor, coupled across the secondary winding as a function of the energy transfer. The voltage across the output capacitor may provide the DC output voltage of the switching power supply.
In many lighting applications, it may be desirable to dim the output of a solid state lighting apparatus. Conventional AC dimming circuits operate using a “phase cut” technique in which portions of the cycles of an AC power signal, such as the leading edge and/or trailing edge are suppressed. The corresponding reduction of energy in the power signal reduces the total luminescent power output by a conventional incandescent light source, causing the light to dim. However, because a solid state lighting apparatus may consume significantly less power than an incandescent device, there may be problems associated with dimming a solid state lighting apparatus in this manner.
For example, for operation of a leading edge, trailing edge, or electronic low voltage (ELV) dimmer on an electronic power supply for the purpose of dimming LEDs, the power consumed by the power supply should be sufficient to ensure proper operation of the dimmer. Insufficient AC line current will commutate (shut off) a triac based dimmer, and may cause erratic operation of an ELV dimmer.